The IEEE 802.11 WLAN standard provides a transmission rate of 11 Mbps (IEEE 802.11b) and 54 Mbps (IEEE 802.11a) based on an unlicensed band at 2.4 GHz or 5 GHz. In more detail, the IEEE 802.11g provides a transmission rate of 54 Mbps by using orthogonal frequency division multiplexing (OFDM) at 2.4 GHz, and the IEEE 802.11n provides a transmission rate of 300 Mbps for four spatial streams by using multiple input multiple output-OFDM (MIMO-OFDM). The IEEE 802.11n supports a channel bandwidth of 40 MHz, and in this case, provides a transmission rate of 600 Mbps.
A TV white space includes a very high frequency (VHF) band (54˜72 MHz, 76˜88 MHz, 174˜216 MHz) and an ultra high frequency (UHF) band (470˜698 MHz) which are allocated to a broadcast TV, and means a frequency band where use of an unlicensed device is allowed on the condition that communication of a licensed device (TV broadcasting and radio mike, etc.) operating at the corresponding frequency band is not interrupted.
However, in the following description, the TV white space is only an example of a white space, and the white space may refer to a TV white space and a random band/communication zone where an unlicensed device may be operated while protecting a licensed device.
An unlicensed device which desires to use the aforementioned white space should provide a protection function for a licensed device. Accordingly, the unlicensed device should necessarily check whether the licensed device occupies a white space band, before starting to transmit a signal at the white space band (for example, TV band). To this end, it is preferable that the unlicensed device acquires available channel list information at a corresponding zone by accessing a geo-location database through Internet or dedicated network. In this case, the geo-location database means a database that stores and manages information of a licensed device registered therewith and channel use information varied dynamically depending on geo-location and use time of licensed devices.
Also, a station (hereinafter, referred to as “STA”) for operating at a white space may perform spectrum sensing mechanism. Examples of the spectrum sensing mechanism may include an energy detection mechanism and a feature detection mechanism. In other words, as a spectrum sensing result of the STA, if strength of a received signal at a corresponding frequency is more than a given value, it may be determined that the corresponding frequency is used by the licensed device (or incumbent user), or if a DTV preamble is detected in case of a TV white space, it may be determined that the incumbent user is using a corresponding channel.
And, if it is determined that the incumbent user is using a channel directly adjoining a channel currently in service, the STA needs to lower a transmission power.